Photodiode and method of fabricating photodiode

ABSTRACT

A light-absorbing layer is composed of a compound-semiconductor film of chalcopyrite structure, a surface layer is disposed on the light-absorbing layer, the surface layer having a higher band gap energy than the compound-semiconductor film, an upper electrode layer is disposed on the surface layer, and a lower electrode layer is disposed on a backside of the light-absorbing layer in opposition to the upper electrode layer, the upper electrode layer and the lower electrode layer having a reverse bias voltage applied in between to detect electric charges produced by photoelectric conversion in the compound-semiconductor film, as electric charges due to photoelectric conversion are multiplied by impact ionization, while the multiplication by impact ionization of electric charges is induced by application of a high-intensity electric field to a semiconductor of chalcopyrite structure, allowing for an improved dark-current property, and an enhanced efficiency even in detection of low illumination intensities, with an enhanced S/N ratio.

CROSS REFERENCE TO RELATED APPLICATIONS AND INCORPORATION BY REFERENCE

This application is based upon and claims the benefits of priority fromprior Japanese Patent Application Nos. P2009-120769 and P2009-297205filed on May 19, 2009, and Dec. 28, 2009, respectively, the entirecontents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a photodiode and a fabrication methodthereof, and particularly, to a photodiode including acompound-semiconductor film of chalcopyrite structure and adapted forfavorable wavelength sensitivity, low dark current, and low-voltageoperation, and a fabrication method thereof.

BACKGROUND ART

There have been thin-film solar cells including a light-absorbing layerusing: CuInSe₂ being a compound-semiconductor film of a chalcopyritestructure composed of a Group-Ib, a Group-IIIb, and a Group-VIb element(that is a CIS-based film); or Cu(In, Ga)Se₂ being a solid solutionthereof with Ga as a solute (that is a CIGS-based film). They have beenadapted for high efficiencies of energy conversion with reduceddeteriorations of efficiency such as due to light irradiation, as anadvantage.

There have been literatures (cf. Patent Literature 1 and PatentLiterature 2) disclosing photoelectric conversion devices employing acompound-semiconductor film of chalcopyrite structure with a greatlyreduced dark current, and fabrication methods thereof.

Also, there have been literatures (cf. Patent Literature 3 and PatentLiterature 4) disclosing methods of formation such as by selenidation ofa high-quality CIGS-based film.

Also, there has been a literature (cf. Non-Patent Literature 1)disclosing effects of surface Cu_(2-x)Se layers on growth and propertiesof CuInSe₂ films.

CITATION LIST

-   Patent Literature 1: Japanese Patent Application Laying-Open    Publication No. 2007-123720-   Patent Literature 2: Japanese Patent Application Laying-Open    Publication No. 2007-123721-   Patent Literature 3: U.S. Pat. No. 5,436,204-   Patent Literature 4: U.S. Pat. No. 5,441,897-   Non-Patent Literature 1: S. Niki, et al.: “Effects of the surface    Cu_(2-x)Se phase on the growth and properties of CuInSe₂ films”,    APPLIED PHYSICS LETTERS, VOLUME 74, Number 11, 15 MARCH 1999, pp.    1630-1632

SUMMARY OF THE INVENTION Technical Problem

CIS-based films and CIGS-based films have their applications nowmainstreaming to solar cells.

The inventors focused attentions on those compound-semiconductor filmmaterials having high light-absorption coefficients, as well as theircharacteristic high-sensitivities over wide bands ranging from visiblewavelengths to near-infrared wavelengths, and examined use of them as aphotodiode. As an issue on the way, there was a conductive CuSe layerappearing on a surface of once-made CIGS-based film, causing anincreased dark current.

It is an object of the present invention to provide a photodiode and afabrication method thereof, the photodiode being adapted for a favorablecharacteristic of wavelength sensitivity, with a suppressed Cuprecipitation on a surface of as-made CIGS film, allowing for lowdark-current and low-voltage operation.

Solution to Problem

To achieve the object, according to an aspect of the present invention,there is a photodiode comprising a light-absorbing layer composed of acompound-semiconductor film of chalcopyrite structure, a surface layerdisposed on the light-absorbing layer, the surface layer having a higherband gap energy than the compound-semiconductor film, an upper electrodelayer disposed on the surface layer, and a lower electrode layerdisposed on a backside of the light-absorbing layer in opposition to theupper electrode layer, wherein the upper electrode layer and the lowerelectrode layer have a reverse bias voltage applied in between to detectelectric charges produced by photoelectric conversion in thecompound-semiconductor film of chalcopyrite structure.

According to another aspect of the present invention, there is aphotodiode comprising a light-absorbing layer composed of acompound-semiconductor film of chalcopyrite structure, a surface layerdisposed on the light-absorbing layer, the surface layer having a higherband gap energy than the compound-semiconductor film, a buffer layerdisposed on the surface layer, an upper electrode layer disposed on thebuffer layer, and a lower electrode layer disposed on a backside of thelight-absorbing layer in opposition to the upper electrode layer,wherein the upper electrode layer and the lower electrode layer have areverse bias voltage applied in between to detect electric chargesproduced by photoelectric conversion in the compound-semiconductor filmof chalcopyrite structure.

According to another aspect of the present invention, there is a methodof fabricating a photodiode, the method comprising a first step ofholding a substrate temperature at a first temperature, retaining acomposition ratio of (Cu/(In+Ga)) to a 0 in a Group-III elementexcessive state, a second step of holding the substrate temperaturewithin a range from the first temperature to a second temperature higherthan the first temperature, shifting the composition ratio of(Cu/(In+Ga)) to a 1.0 or more being a Cu element excessive state, and athird step of shifting the composition ratio of (Cu/(In+Ga)) from the1.0 or more being the Cu element excessive state to a 1.0 or less beinga Group-III element excessive state, wherein the third step comprisesholding the substrate temperature for a first period at the secondtemperature, and holding the substrate temperature for a second periodwithin a range from the second temperature to a third temperature lowerthan the second temperature, for a low-temperature growth to form acompound-semiconductor film of chalcopyrite structure with a suppressedsurface precipitation of Cu.

Advantageous Effects of Invention

The present invention affords to provide a photodiode and a fabricationmethod thereof, the photodiode being adapted for a favorablecharacteristic of wavelength sensitivity, with a suppressed Cuprecipitation on a surface of as-made CIGS film, allowing for lowdark-current and low-voltage operation.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a schematic plan view of pattern configuration of aphotodiode according to an embodiment of the present invention.

FIG. 1B is a schematic sectional view of structure along line I-I ofFIG. 1A.

FIG. 2 is a graph representing a relationship between electrical fieldintensities and depthwise distances for a reverse bias voltage V_(R) (V)applied between an upper electrode layer and a lower electrode layer ofa photodiode according to the embodiment.

FIG. 3 is a graph of schematic current vs. voltage characteristic curvesdescribing a multiplication between reception and non-reception of lighton photodiodes according to the embodiment.

FIG. 4 is a graph of curves each representing a relationship betweencurrent densities I_(A) (A/cm²) and reverse bias voltages V_(R) (V)applied between an upper electrode layer and a lower electrode layer, asa characteristic of photodiodes according to the embodiment.

FIG. 5 is a graph plotting relationships between multiplication factorsM and reverse bias voltages V_(R) (V) for parametric illuminationintensities of received light, as characteristics of photodiodesaccording to the embodiment.

FIG. 6A is a schematic sectional view of structure of a photodiodeaccording to the embodiment.

FIG. 6B is a schematic sectional view of structure of a photodiode as acomparative example.

FIG. 7A is a time chart showing steps for formation of acompound-semiconductor film of chalcopyrite structure, coveringsubstrate temperatures and elements as ingredients for film formation bythe steps, in a method of fabricating a photodiode according to theembodiment.

FIG. 7B is a time chart showing ratios of (Cu/(In+Ga)) in the steps ofFIG. 7A.

FIG. 8 is a graph plotting relationships between current densities(A/cm²) and reverse bias voltages V_(R) (V), as a combination ofcharacteristics of the photodiode according to the embodiment (FIG. 6A)and characteristics of the photodiode as a comparative example (FIG.6B).

FIG. 9 is a graph plotting results of an SIMS analysis of the photodiodeas a comparative example (FIG. 6B).

FIG. 10 is a graph plotting results of an SIMS analysis of thephotodiode according to the embodiment (FIG. 6A).

FIG. 11 is a graph plotting band gap energies Eg (eV) with an increasingtendency to a decreasing concentration of Cu in surface layer, as acharacteristic of photodiodes according to the embodiment.

FIG. 12A is a graph plotting a relationship between dark currentdensities I_(d) (A/cm²) and surface layer depths d, as a characteristicof photodiodes according to the embodiment.

FIG. 12B is a schematic sectional view of a structure of photodiodesaccording to the embodiment.

FIG. 13 is a graph of quantum efficiency vs. wavelength characteristicsfor parametric surface layer depths d of photodiodes according to theembodiment.

FIG. 14 is a graph of quantum efficiency vs. wavelength characteristicsof photodiodes according to the embodiment.

FIG. 15 is a graph of light absorption characteristics of photodiodesaccording to the embodiment.

FIG. 16 is a graph plotting a relationship between band gap energies Eg(eV) and composition ratios of In/(In+Ga) in compound-semiconductorfilms of chalcopyrite structure of photodiodes according to theembodiment

FIG. 17 is a graph plotting a relationship between dark currentdensities I_(d) (A/cm²) and composition ratios of Ga/(Ga+In) of surfacelayer in photodiodes according to the embodiment

FIG. 18A is an example of SEC micrograph in section of a photodiodeformed with a surface layer according to the embodiment.

FIG. 18B is an explanatory diagram of structure corresponding to FIG.18A.

FIG. 19 is a graph plotting results of an SIMS analysis of thephotodiode according to the embodiment (FIG. 18A).

FIG. 20 is a graph plotting relationships between forward and reversecurrent densities (A/cm²) and bias voltages V (V), as characteristics ofthe photodiode according to the embodiment (FIG. 18A).

DESCRIPTION OF EMBODIMENTS

There will be described an embodiment of the present invention withreference to the drawings. In the drawings, identical or similarelements or portions are designated at identical or similar referencenumbers. The drawings are schematic with illustrativethickness-to-planer dimension relationships, inter-layer thicknessratios, etc, as they are non-specific. For specific dimensions such asthickness, associated description should be taken into account fordetermination. Also, dimensional relationships and ratios may bedifferent between drawings.

The above-noted embodiment is illustrative as a device and/or a methodto implement a technical concept of the present invention, and notrestrictive in material quality, shape, structure, arrangement, etc. Theembodiment may well be changed or modified in various manners withoutdeparting from the scope of appended claims.

Embodiment

FIG. 1A shows a schematic plan of a photodiode according to theembodiment, in the form of a pattern configuration, and FIG. 1B, aschematic section of structure along line I-I of FIG. 1A.

Referring to FIG. 1A and FIG. 1B, according to the embodiment, thephotodiode includes: a light-absorbing layer 12 composed of acompound-semiconductor film of chalcopyrite structure; a surface layer14 disposed on the light-absorbing layer 12, the surface layer 14 havinga higher band gap energy than the compound-semiconductor film; an upperelectrode layer 18 disposed on the surface layer 14; and a lowerelectrode layer 10 disposed on a backside of the light-absorbing layer12 in opposition to the upper electrode layer 18. It is configured forthe upper electrode layer 18 and the lower electrode layer 10 to have areverse bias voltage V_(R) (V) applied in between to detect electriccharges produced by photoelectric conversion in thecompound-semiconductor film of chalcopyrite structure.

According to the embodiment, the photodiode thus has a reverse biasvoltage V_(R) (V) applied between the upper electrode layer 18 and thelower electrode layer 10, which causes electric charges produced byphotoelectric conversion to be multiplied by impact ionization in thelight-absorbing layer 12 composed of a compound-semiconductor film ofchalcopyrite structure.

The upper electrode layer 18 is formed as a transparent electrode. As amaterial for the transparent electrode, there may be use of, forinstance, a ZnO film, an ITO film, a tin oxide (SnO₂) film, or an indiumoxide (In₂O₃) film.

For the lower electrode layer 10, there may be use of, for instance,molybdenum (Mo), niobium (Nb), tantalum (Ta), tungsten (W), etc.

For the light-absorbing layer 12 of the photodiode shown in FIG. 1B, thecompound-semiconductor film of chalcopyrite structure is made of Cu(In_(x), Ga_(1-x)) Se₂ (0≦x≦1).

The surface layer 14 can be formed as an i-type CIGS layer, forinstance. It is made as a high-resistivity layer with a lower Cuconcentration than the light-absorbing layer 12 made of Cu (In_(x),Ga_(1-x)) Se₂ (0≦x≦1). That is, the surface layer 14 is formed as ahigh-resistivity layer by way of low-temperature growth forconfiguration with a suppressed Cu precipitation on a surface of as-madeCIGS film.

There may be a surface layer 14 made of In_(x), Ga_(1-x)Se (0≦x≦1)completely free of Cu.

The upper electrode layer 18 may be configured with a non-doped ZnO(i-ZnO) film disposed on the surface layer 14, and an n-type ZnO filmformed on the non-doped ZnO (i-ZnO) film.

According to the embodiment, there may be a photodiode additionallyincluding a buffer layer 16 disposed on the upper surface layer 14, withthe upper electrode layer 18 disposed on the buffer layer 16.

The buffer layer 16 has an opposite conductivity type to thelight-absorbing layer 12.

As a material for formation of the buffer layer 16, there may be use ofCdS, ZnS, ZnO, ZnMgO, ZnSe, In₂S₃, etc.

The upper electrode layer 18 may well be provided as a transparentelectrode of a non-doped ZnO (i-ZnO) film, to plug voids or pinholes ina substrate CIGS film with a semi-insulating layer, thus preventingleakage. The non-doped ZnO (i-ZnO) film may then have an increased depthto reduce dark currents at a p-n junction boundary surface.

According to the embodiment, the configuration of photodiode with abuffer layer 16 disposed on the surface layer 14 allows for reduction ofleak currents by the buffer layer 16, while the present invention is notrestricted thereto. For instance, there may be use of a buffer-lessconfiguration to provide an upper electrode layer 18 on the surfacelayer 14.

Further, as illustrated in FIG. 1B, there may be a mesa structure inconfiguration of part of surface layer 14, buffer layer 16, and upperelectrode layer 18. Provision of such a mesa structure allows for asecured reverse breakdown voltage nature and a reduced reverse leakcurrent of photodiodes according to the embodiment.

There may be photodiodes configured as a photo sensor with a sensitivityrange covering a near-infrared light region, as well, according to theembodiment illustrated in FIGS. 1A, and 1B.

According to the embodiment, there may well be photodiodes implementedwith: an upper electrode layer 18 approximately 500 nm thick forinstance; a buffer layer 16 within a thickness range of approximately 50nm to 100 nm for instance; a surface layer 14 within a depth range ofapproximately 200 nm to 600 nm for instance; a light-absorbing layer 12within a thickness range of approximately 1 μm to 2 μm for instance; alower electrode layer 10 approximately 600 nm thick for instance; and atotal structure approximately 3 μm thick for instance, with the lowerelectrode layer 10 and the upper electrode layer 18 inclusive.

(Multiplication Mechanism of Photodiode)

FIG. 2 represents a relationship between electrical field intensitiesand depthwise distances for a reverse bias voltage V_(R) (V) appliedbetween an upper electrode layer 18 and a lower electrode layer 10 of aphotodiode according to the embodiment. With the reverse bias voltageV_(R) (V) applied between upper electrode layer 18 and lower electrodelayer 10, the photodiode had a buffer layer 16 and a light-absorbinglayer 12 substantially depleted, and an electric field with intensitiesas high as approximately 1×10⁴ (V/cm) or near, for instance, applied toa surface layer 14 formed as a high-resistivity layer. The intensity ofelectric field may have a peak varied within a range of approximately1×10⁴ to 4×10⁵ (V/cm) or near, for instance, depending on a film depthand a CIGS composition of the light-absorbing layer 12.

In photodiodes according to the embodiment, there were phenomena ofAvalanche multiplication developed each with a low voltage within arange of approximately 12 V or near applied as a reverse bias voltageV_(R) (V) between an upper electrode layer 18 and a lower electrodelayer 10.

For typical silicon devices, there are levels of approximately 100 V ornear needed to cause an Avalanche multiplication.

FIG. 3 describes, in a graph of schematic current vs. voltagecharacteristic curves, a multiplication between reception andnon-reception of light on a photodiode according to the embodiment.

As will be apparent from FIG. 3, there were little variations indifference between a region of values P1 of dark current in anirradiation-less phase and a region of values P2 of photo current in anirradiated phase, along with application of reverse bias voltages V_(R)(V) developing electric fields of relatively low intensities. To thecontrary, there were very significant variations in difference between aregion of values A1 of dark current in the irradiation-less phase and aregion of values A2 of photo current in the irradiated phase, along withapplication of reverse bias voltages V_(R) (V) developing electricfields of relatively high intensities to be sufficient to induce anAvalanche multiplication effect. Accordingly, there were apparentimprovements including an S/N ratio of the photodiode according to theembodiment.

FIG. 4 shows, in a graph, those curves representing relationshipsbetween current densities I_(A) (A/cm²) and reverse bias voltages V_(R)(V) applied between an upper electrode layer 18 and a lower electrodelayer 10, as characteristics of a photodiode according to theembodiment. The solid-line curve represents data of measurements ofphoto current density for a light irradiation of 1,000 Lux, and thedotted-line curve represents data of measurements of dark currentdensity. As will be apparent from measurement data of FIG. 4 also, therewere little variations in difference between a region of dark current inan irradiation-less phase and a region of photo current under the lightirradiation of 1,000 Lux, along with application of reverse biasvoltages V_(R) (V) developing electric fields of relatively lowintensities. To the contrary, there were very significant variations indifference between a region of dark current in the irradiation-lessphase and a region of photo current under the light irradiation of 1,000Lux, along with application of reverse bias voltages V_(R) (V)developing electric fields of relatively high intensities to besufficient to induce an Avalanche multiplication effect. According tothe embodiment, the photodiode was adapted for a low-voltage operation,which made levels of dark current substantially comparable between theregion thereof along with application of reverse bias voltages V_(R) (V)developing electric fields of relatively low intensities and the regionthereof along with application of reverse bias voltages V_(R) (V)developing electric fields of relatively high intensities to besufficient to induce an Avalanche multiplication effect. Accordingly,there were observed improvements including an S/N ratio of thephotodiode according to the embodiment.

FIG. 5 shows, in a graph, plots of relationships between multiplicationfactors M and reverse bias voltages V_(R) (V) for parametricillumination intensities of irradiated light, as characteristics of aphotodiode according to the embodiment. As will be seen from FIG. 5,according to the embodiment, the photodiode was adapted to operate withreverse bias voltages V_(R) (V) as relatively low as 0 V to −10 V. Itwill also be understood that the lower the illumination intensity thehigher the multiplication factor M. For instance, at a reverse biasvoltage V_(R) (V)=−10 V, there were multiplication factors of 100 to 20or near obtained under illumination intensities within a range of 1 Luxto 1,000 Lux.

(Process of Forming a Compound-Semiconductor Film of ChalcopyriteStructure)

In this embodiment, photodiodes are configured with acompound-semiconductor film of chalcopyrite structure adapted to work asa light-absorbing layer 12 thereof, which may well be formed on, amongothers, a semiconductor substrate or a glass substrate by way of: a PVD(Physical Vapor Deposition) method, such as by a vacuum evaporation orby a sputtering; or an MBE (Molecular Beam Expitaxy) method. Here, thePVD method collectively refers to a set of applicable methods includinga method of depositing a vacuum-vaporized raw material to make a film.

For use of the vacuum evaporation method, there may be a set of vacuumevaporation sources adapted for individual evaporation of compoundcomponents (Cu, In, Ga, Se, S) on a substrate.

For the sputtering method, there may be use of a chalcopyrite compoundas a target, or use of components thereof as individual targets.

For formation of compound-semiconductor films of a chalcopyritestructure on a glass substrate, the substrate may be heated to hightemperatures causing composition changes due to defection of achalcogenide element. In such a case, as-made films may well beheat-treated in a Se or S vapor atmosphere at temperatures of 400 to 600degrees C. for a period within a range of one to several hours (for aselenidation or sulfurization process) to supplement Se or S.

(Fabrication Method)

Description is now made of a process for formation of acompound-semiconductor film of chalcopyrite structure, with reference toFIG. 7A and FIG. 7B, in course of a fabrication of photodiodes accordingto the embodiment, of which an example of configuration will bediscussed later on with reference to FIG. 6A, FIG. 63, and FIG. 8 etseq. FIG. 7A shows steps of that process, covering substratetemperatures and elements as ingredients by the steps, and FIG. 7B showscomposition ratios of (Cu/(In+Ga)) in the steps.

According to the embodiment, the fabrication of photodiodes isimplemented as a photodiode fabrication method including a filmformation process three-stepped as illustrated in FIG. 7A and FIG. 7Bfor formation of a compound-semiconductor film of chalcopyritestructure.

The film formation process is divided into three steps being a firststep, a second step, and a third step as illustrated in FIG. 7A and FIG.73 as an example for formation of a p-type CIGS filmcomposition-controlled to be Cu (In_(x), Ga_(1-x)) Se₂ (0≦x≦1). In thisregard, FIG. 7A shows substrate temperatures and component elements tobe vacuum-evaporated for formation of the film. There may be use of asputtering method for the film formation. FIG. 7B shows ratios of(Cu/Group-III (In+Ga)).

As illustrated in FIG. 7A and FIG. 7B, according to the embodiment, thephotodiode fabrication method includes: a first step (corresponding to afirst interval of time designated by a section 1 a) of holding asubstrate temperature at a first temperature T1, retaining a compositionratio of (Cu/(In+Ga)) to a 0 in a Group-III element excessive state; asecond step (corresponding to a second interval of time designated by asection 1 b) of holding the substrate temperature within a range fromthe first temperature T1 to a second temperature T2 higher than thefirst temperature T1, shifting the composition ratio of (Cu/(In+Ga)) toa 1.0 or more being a Cu element excessive state; and a third step(corresponding to a third interval of time designated by combination ofa section 3 a and a section 3 b) of shifting the composition ratio of(Cu/(In+Ga)) from the 1.0 or more being the Cu element excessive stateto a 1.0 or less being a Group-III element excessive state, wherein thethird step (corresponding to the third interval of time) is composed ofholding the substrate temperature for a first period (as the section 3a) at the second temperature T2, and holding the substrate temperaturefor a second period (as the section 3 b) within a range from the secondtemperature T2 to a third temperature T3 lower than the secondtemperature T2, for a low-temperature growth to form acompound-semiconductor film of chalcopyrite structure with a suppressedsurface precipitation of Cu.

The third step has the sub-step of holding the substrate temperature forthe second period (as the section 3 b) within the range from the secondtemperature T2 to the third temperature T3 lower than the secondtemperature T2, which allows for a low-temperature growth to form ahigh-resistivity surface layer 14 with a suppressed Cu precipitation ona surface of the compound-semiconductor film.

The compound-semiconductor film of chalcopyrite structure may well bemade of Cu (In_(x), Ga_(1-x)) Se₂ (0≦x≦1).

The third temperature may well coincide within a range of 300 degrees C.or more and 400 degrees C. or less.

The second temperature may well be 550 degrees C. or less.

The third step may well have a ratio of (Cu/(In+Ga)) within a range of0.5 to 1.3 at an end of the first period (as the section 3 a), and aratio of (Cu/(In+Ga)) of 1.0 or less at an end of the second period (asthe section 3 b).

According to the embodiment, the photodiode fabrication method includesthe third step divided into two sub-steps being: the section 3 a for ahigh-temperature processing at the substrate temperature T2; and thesection 3 b for a shift to a low-temperature processing at the substratetemperature T3, for positive formation of the surface layer 14 to bemade as an i-type CIGS layer on a surface of the compound-semiconductorfilm. The substrate temperature T3 should preferably be within a rangeof 300 to 400 degrees C., and specifically, approximately 300 degreesC., for instance.

According to the embodiment, the photodiode fabrication method hasfilm-constituent elements vacuum-evaporated not simultaneously, butthrough the three steps, thus permitting the intra-film distribution ofconstituent elements to be controlled to some extent. There is flux ofbeam of In as well as Ga used for a band gap control of thecompound-semiconductor film. There may be use of a ratio of(Cu/Group-III (In+Ga)) for control of Cu concentration in a CIGS film.There are resultant merits including a facilitated setting of(Cu/Group-III (In+Ga)) ratio, an easy film depth control, and a securedconstant supply of Se.

The facilitated setting of (Cu/Group-III (In+Ga)) ratio permits theratio of (Cu/Group-III (In+Ga)) to be reduced in the third step to forma surface layer 14 of an i-type CIGS on a surface ofcompound-semiconductor film, with an enhanced film depthcontrollability, in a facilitated manner. Such the i-type CIGS surfacelayer 14 appears working as an i-layer with a decreased carrier numberdue to a low concentration of Cu controlling intra-film carrierconcentrations.

It is noted that the photodiode fabrication method described has beenaddressed to a low-temperature processing in the section 3 b illustratedin FIGS. 7A and 7B, while the present invention is not restrictedthereto. For instance, the process may be once interrupted past a thirdstep 3 a, for a pause followed by a consecutive temperature shift tosuch a temperature as shown at the section 3 b in FIG. 7A, accompaniedby a decreasing Cu proportion to Group-III elements, to form a desirableCIGS surface layer. Further, the present invention is not restricted tothe three-stepped method described. For instance, there may be use of abi-layer method to implement the invention. The bi-layer method is amethod of forming a CIGS layer such as by a vacuum deposition or asputtering, including a first step of using four elements being Cu, In,Ga, and Se, and a consecutive second step of using three elements beingIn, Ga, and Se, excluding Cu. There may be a film formed by the bi-layermethod, and followed by a consecutive temperature shift to such atemperature as shown at the section 3 b in FIG. 7A, accompanied by adecreasing Cu proportion to Group-III elements, to form a desirable CIGSsurface layer. The present invention may be implemented by use of anyother film formation method available (e.g. a sulfurization method, aselenidation-sulfurization method, a simultaneous vacuum depositionmethod, an in-line simultaneous vacuum deposition method, a fastsolid-phase selenidation method, an RR (roll-to-roll) method, an ionizeddeposition RR method, a simultaneous vacuum deposition RR method, anelectro-deposition method, a hybrid process, a hybrid sputtering RRmethod, a nanoparticle printing method, a nanoparticle printing RRmethod, or the FASST® process) for formation of a CIGS film followed bysuch a low-temperature film-forming step as described.

According to the embodiment, the photodiode fabrication method may havea three-stepped process including a third step of decreasing a(Cu/Group-III (In+Ga)) ratio to form a surface layer 14 of an i-typeCIGS on a surface of compound-semiconductor film, with an enhanced filmdepth controllability, in a facilitated manner, permitting the i-typeCIGS surface layer 14 to be formed thinner with ease.

FIG. 6A shows in schematic section a structure of a photodiode accordingto the embodiment, and FIG. 6B shows in schematic section a structure ofa photodiode as a comparative example. The comparative example shown inFIG. 6B is configured for use as a solar cell.

In FIG. 6B, the comparative example has a layer configuration made ofZnO (designated by reference number 18)/Cds (designated by referencenumber 16)/CIGS (designated by reference number 12)/Mo (designated byreference number 10). On the other hand, in FIG. 6A, the photodiodeaccording to the embodiment has a layer configuration made of ZnO(designated by reference number 18)/Cds (designated by reference number16)/InGaSe (designated by reference number 14)/CIGS (designated byreference number 12)/Mo (designated by reference number 10).

FIG. 8 shows, in a graph, plots of relationships between currentdensities (A/cm²) and reverse bias voltages V_(R) (V), as a combinationof characteristics of the photodiode according to the embodiment (FIG.6A) and characteristics of the photodiode as a comparative example (FIG.6B). According to the embodiment, the photodiode in FIG. 6A had asurface layer 14 made as a high-resistivity, high-energy band gap layerby a compound-semiconductor film formed with a reduced (Cu/Group-III(In+Ga)) ratio, with a resultant effect enabling an enhancement ofcurrent density (A/cm²) as much as approximately four figures underapplication of a reverse bias voltage V_(R) (V).

FIG. 9 shows, in a graph, plots of data of an SIMS analysis of thephotodiode as a comparative example (FIG. 6B), and FIG. 10 shows, in agraph, plots of data of an SIMS analysis of the photodiode according tothe embodiment (FIG. 6A).

For the photodiode as a comparative example, there was acompound-semiconductor film formed with a history of substratetemperatures of approximately 550 degrees C. or near in both second andthird steps, accompanied by a (Cu/Group-III) ratio of approximately0.75. Plots of SIMS analysis data in FIG. 9 showed no reduction ofsurface Cu concentration observed on the compound-semiconductor film. Inother words, the compound-semiconductor film was not undoped at thesurface.

In this regard, as shown in FIG. 10, the photodiode according to theembodiment had Cu concentrations observed with decreasing tendencieswithin a depth range of approximately 0.5 to 1.0 μm.

For the photodiode according to the embodiment, thecompound-semiconductor film was formed with a history of substratetemperatures of approximately 400 degrees C. or near in both the secondstep and the first period 3 a of the third step, and those ofapproximately 300 degrees C. or near in the second period 3 b of thethird step, accompanied by a (Cu/Group-III) ratio of 0.75, to have Cuconcentrations observed with significant decreasing tendencies at asurface region of the compound-semiconductor film. Thecompound-semiconductor film thus had an undoped surface layer 14 formedas an i-type CIGS layer. It is noted that the second step had substratetemperatures of approximately 400 degrees C. or near, accompanied by a(Cu/Group-III) ratio of 0.92, and the second period 3 b of the thirdstep had substrate temperatures of approximately 300 degrees C. or near,accompanied by the (Cu/Group-III) ratio of 0.75. As will be seen fromFIG. 10, the undoped surface layer 14 (as an i-type CIGS layer) of thecompound-semiconductor film had depths of approximately 300 nm or near.

FIG. 11 shows, in a graph, plots of band gap energies Eg (eV) with anincreasing tendency to a decreasing concentration of Cu, as acharacteristic of surface layers of photodiodes according to theembodiment. The photodiodes were each configured with: a light-absorbinglayer 12 formed with a CIGS layer, which had a band gap energy Eg ofapproximately 1.25 eV; and a surface layer 14 formed on thelight-absorbing layer 12, with a history of substrate temperaturesincluding a third temperature T3 (cf. FIG. 7A), which was varied. For atemperature T3 of approximately 400 degrees C., the surface layer 14 hada band gap energy raised to approximately 1.35 eV. Still more, for atemperature T3 of approximately 300 degrees C., the surface layer 14 hada band gap energy raised to approximately 1.39 eV. There were band gapenergies Eg observed with a tendency to increase with a decrease in Cuconcentration of the surface layer 14.

FIG. 12A shows, in a graph, plots of a relationship between dark currentdensities I_(d) (A/cm²) and surface layer depths d, as a characteristicof photodiodes according to the embodiment. FIG. 12B shows in schematicsection a structure including a light-absorbing layer 12 and a surfacelayer 14 of photodiodes having the characteristic shown in FIG. 12A.

The light-absorbing layer 12 had a CIGS layer formed by a growthtemperature of 400 degrees C. with a composition ratio of Ga/(Ga+In) of0.4. The surface layer 14 had an In_(0.6)Ga_(0.4)Se layer formed by agrowth temperature of 300 degrees C. with a composition ratio ofGa/(Ga+In) of 0.4. There were dark current densities I_(d) (A/cm²)observed with a tendency to gradually decrease with an increasing depthd, as the depth of surface layer 14 was varied from 0 to 1,300 nm ornear. For instance, there was a dark current density I_(d) (A/cm²) ofapproximately 3×10⁻¹⁰ (A/cm²) observed with a depth of approximately1,300 nm.

FIG. 13 shows, in a graph, quantum efficiency vs. wavelengthcharacteristics for parametric surface layer depths d of photodiodesaccording to the embodiment. The photodiodes were each configured with:a light-absorbing layer 12 including a CIGS layer formed by a growthtemperature of 400 degrees C. with a composition ratio ofGa/(Ga+In)=0.4; and a surface layer 14 including an In_(0.6)Ga_(0.4)Selayer formed by a growth temperature of 300 degrees C with a compositionratio of Ga/(Ga+In)=0.4. As shown in FIG. 13, there were quantumefficiencies observed over a wide wavelength range with a tendency toincrease as the surface layer 14 had a reduced depth d. The thinner thedepth d of surface layer 14, the smaller the light absorption of surfacelayer 14 on the shorter wavelength side, and the higher the quantumefficiency, as an observed tendency.

(Photoelectric Conversion Characteristic)

FIG. 14 shows, in a graph, quantum efficiency vs. wavelengthcharacteristics of photodiodes according to the embodiment. Thephotodiodes were each configured with a light-absorbing layer 12 made ofa compound-semiconductor film of chalcopyrite structure (Cu (In_(x),Ga_(1-x)) Se₂ (0≦x≦1)) working as a layer to absorb light, exhibiting,as reflection of its quantum efficiency, photoelectric conversioncharacteristics with high quantum efficiencies over a wide wavelengthband encompassing from visible wavelengths to near-infrared wavelengths.Quantum efficiencies were doubled relative to silicon (Si)-basedphotoelectric conversion characteristics. The compound-semiconductorfilm of chalcopyrite structure, working as a light-absorbing layer 12,may have its composition (Cu (In_(x), Ga_(1-x)) Se₂ (0≦x≦1)) changedbetween Cu(InGa)Se₂ and Cu(In)Se₂ to extend the wavelength band up tonear-infrared wavelengths approximately 1,300 nm.

(Light Absorption Characteristic)

FIG. 15 shows, in a graph, light absorption characteristics ofphotodiodes according to the embodiment. The photodiodes were eachconfigured with a compound-semiconductor film of chalcopyrite structure(Cu (In_(x), Ga_(1-x)) Se₂ (0≦x≦1)) working as a light-absorbing layer12, exhibiting, as reflection of its light absorption coefficient, arobust light absorption characteristic over a wide wavelength bandencompassing from visible wavelengths to near-infrared wavelengths.

For instance, it had a silicon (Si)-based absorption coefficient timesapproximately 100 even in a visible wavelength region. Thecompound-semiconductor film of chalcopyrite structure, working as alight-absorbing layer 12, may have its composition (Cu(In_(x), Ga_(1-x))Se₂ (0≦x≦1)) changed between Cu(InGa)Se₂ and Cu(In)Se₂ to extend theabsorption characteristic up to wavelengths approximately 1,300 nm ornear.

(Characteristics on Band Gap Energy and In/(In+Ga) Composition Ratio)

FIG. 16 shows, in a graph, plots of a relationship between band gapenergies Eg (eV) and composition ratios of In/(In+Ga) incompound-semiconductor films of chalcopyrite structure (Cu(In_(x),Ga_(1-x)) Se₂ (0≦x≦1)) of photodiodes according to the embodiment.

As will be seen from FIG. 16, the photodiodes had: a film of Cu (Ga)Se₂with a band gap energy of 1.68 eV; a film of Cu (In, Ga)Se₂ with a bandgap energy of 1.38 eV; and a film of Cu (In)Se₂ with a band gap energyof 1.04 eV, respectively.

Compound-semiconductor films of chalcopyrite structure (Cu(In_(x),Ga_(1-x)) Se₂ (0≦x≦1)) have their In/(In+Ga) composition ratios, whichare controllable to change their band gap energies as shown in FIG. 16,so their photo-electric conversion wavelengths are variable bycomposition ratio control. For instance, there may be acompound-semiconductor film rendered Ga-excessive at an upside of a CIGSfilm therein, as well as near a downside thereof, to provide anincreased band gap energy for reduction of dark current. Also, there maybe a compound-semiconductor film rendered In-excessive at a prescribeddepth inside a CIGS film thereof, to provide a decreased band gap energyfor enhancement of a photo-electric conversion efficiency at anear-infrared wavelength region.

Further, in formation of a CIGS surface layer discussed with referenceto FIGS. 7A and 73, there may be a Ga content increased in combinationwith a (Cu/Group-III) ratio kept contestant during the surface layerformation, to provide an increased band gap energy on the obverse side,with an enhanced quantum efficiency at the shorter wavelength side.

FIG. 17 shows, in a graph, plots of a relationship between dark currentdensities I_(d) (A/cm²) and composition ratios of Ga/(Ga+In) of surfacelayer 14 in photodiodes according to the embodiment. For instance, therewas a photodiode formed with a surface layer 14 of a Ga/(Ga+In)composition ratio of approximately 0.4, which had a dark current densityId of approximately 3×10⁻³ (A/cm²).

FIG. 18A shows an example of SEM micrograph in section of a photodiodeformed with a surface layer 14 according to the embodiment, and FIG. 18Bshows an explanatory diagram of structure corresponding to FIG. 18A.FIG. 19 shows, in a graph, plots of data of an SIMS analysis of thephotodiode according to the embodiment (FIG. 18A). As will be seen fromFIG. 9, there were Cu concentrations observed as steeply decreasingwithin a depth range of 0 to 0.2 μm, where the surface layer 14 appearsto have had decreased Cu concentrations. FIG. 20 shows, in a graph,plots of relationships between forward and reverse current densities(A/cm²) and bias voltages V (V), as characteristics of the photodiodeaccording to the embodiment (FIG. 18A). There were dark currentdensities of l 0⁻¹⁰ to 10⁻⁶ (A/cm²) appearing at reverse bias voltageswithin a range of 0 to 5 V.

The embodiment described affords a photodiode and a fabrication methodthereof, the photodiode being configured to have high-intensity electricfields applied to a light-absorbing layer including a semiconductor ofchalcopyrite structure, thereby developing a multiplication of electriccharges by impact ionization, with an enhanced dark currentcharacteristic, allowing for a great enhanced efficiency of detectioneven under low irradiation intensities, and an enhanced S/N ratio.

The embodiment described affords a photodiode and a fabrication methodthereof, the photodiode being adapted for a low-temperature growth tosuppress Cu precipitation on a surface of as-made CIGS film, toimplement an enhanced dark current characteristic, thereby permittingobservation of a multiplication that otherwise would not be caused,allowing for detection even under low irradiation intensities.

The embodiment described provides a photodiode and a fabrication methodthereof, the photodiode being adapted for a favorable characteristic ofwavelength sensitivity with suppressed Cu precipitation on a surface ofas-made CIGS film, allowing for low dark-current and low-voltageoperation.

Other Embodiments

While the present invention has been described by a mode of embodiment,it will be construed that the disclosure is not restrictive thereto evenin any discussion or depiction therein. Artisan may associate thedisclosure or part thereof with various alternative embodiment modes,examples, or techniques for application with ease.

According to the embodiment, there are photodiodes as well asfabrication methods thereof each employing Cu (In_(x), Ga_(1-x)) Se₂(0≦x≦1) as a compound-semiconductor film of chalcopyrite structure in alight-absorbing layer. This is not restrictive to the present invention,either.

There are Cu (In_(x), Ga_(1-x)) (Se_(y), S_(1-y)) (0≦x≦1; 0≦y≦1) knowneach as a composition of CIGS film applicable to acompound-semiconductor film. CIGS films of such compositions also areemployable.

There are other compound-semiconductor films of chalcopyrite structure,such as CuAlS₂, CuAlSe₂, CuAlTe₂, CuGaS₂, CuGaSe₂, CuGaTe₂, CuInS₂,CuInSe₂, CuInTe₂, AgAlS₂, AgAlSe₂, AgAlTe₂, AgGaS₂, AgGaSe₂, AgGaTe₂,AgInS₂, AgInSe₂, and AgInTe₂, which also are each applicable.

The embodiment described includes an example of configuration with abuffer layer provided on a compound-semiconductor film (as a CIGS film),and an example of buffer-less configuration with an electrode layerprovided on a compound-semiconductor film (as a CIGS film), while thepresent invention is not limited thereto.

Such, being the case, the present invention covers a variety ofembodiment examples, beyond described ones. For the present invention,the technical matters are defined by characteristic features of appendedclaims to be construed in view of description.

INDUSTRIAL APPLICABILITY

The present invention provides a photodiode with enhanced sensitivitieseven in a region of near-infrared wavelengths, thus having applicationsover a wide wavelength range, such as those to photo detectors, andAvalanche diodes.

REFERENCE SIGNS LIST

-   10 . . . lower electrode layer-   12 . . . light-absorbing layer-   14 . . . surface layer (high-intensity bang gap layer)-   16 . . . buffer layer-   18 . . . upper electrode layer

1. A photodiode comprising: a light-absorbing layer composed of acompound-semiconductor film of chalcopyrite structure, thecompound-semiconductor film of chalcopyrite structure being made of Cu(Inx, Ga1-x) Se2 (0≦x≦1); a surface layer disposed on thelight-absorbing layer, the surface layer having a higher band gap energythan the compound-semiconductor film; an upper electrode layer disposedon the surface layer; and a lower electrode layer disposed on a backsideof the light-absorbing layer in opposition to the upper electrode layer,wherein the upper electrode layer and the lower electrode layer have areverse bias voltage applied in between to detect electric chargesproduced by photoelectric conversion in the compound-semiconductor filmof chalcopyrite structure, wherein the surface layer has a lower Cuconcentration than the compound-semiconductor film made of Cu (Inx,Ga1-x) Se2 (0≦x≦1), and a particle size of the surface layer is smallerthan that of the compound-semiconductor film of chalcopyrite structure.2. The photodiode according to claim 1, wherein the photodiode isconfigured to have electric charges due to photoelectric conversionmultiplied by impact ionization in the compound-semiconductor film ofchalcopyrite structure.
 3. The photodiode according to claim 1, whereinthe upper electrode layer is formed as a transparent electrode.
 4. Thephotodiode according to claim 1, wherein the surface layer is made ofIn_(x), Ga_(1-x)Se (0≦x≦1).
 5. The photodiode according to claim 1,wherein the upper electrode layer comprises: a non-doped ZnO filmprovided on the compound-semiconductor film; and an n-type ZnO filmprovided on the non-doped ZnO film.
 6. A photodiode comprising: alight-absorbing layer composed of a compound-semiconductor film ofchalcopyrite structure, the compound-semiconductor film of chalcopyritestructure being made of Cu (Inx, Ga1-x) Se2 (0≦x≦1); a surface layerdisposed on the light-absorbing layer, the surface layer having a higherband gap energy than the compound-semiconductor film; a buffer layerdisposed on the surface layer; an upper electrode layer disposed on thebuffer layer, and a lower electrode layer disposed on a backside of thelight-absorbing layer in opposition to the upper electrode layer,wherein the upper electrode layer and the lower electrode layer have areverse bias voltage applied in between to detect electric chargesproduced by photoelectric conversion in the compound-semiconductor filmof chalcopyrite structure, wherein the surface layer has a lower Cuconcentration than the compound-semiconductor film made of Cu (Inx,Ga1-x) Se2 (0x≦x≦1), and a particle size of the surface layer is smallerthan that of the compound-semiconductor film of chalcopyrite structure.7. The photodiode according to claim 6, wherein the photodiode isconfigured to have electric charges due to photoelectric conversionmultiplied by impact ionization in the compound-semiconductor film ofchalcopyrite structure.
 8. The photodiode according to claim 6, whereinthe upper electrode layer is formed as a transparent electrode.
 9. Thephotodiode according to claim 6, wherein the surface layer is made ofIn_(x), Ga_(1-x)Se (0≦x≦1).
 10. The photodiode according to claim 6,wherein the upper electrode layer comprises: a non-doped ZnO filmprovided on the compound-semiconductor film; and an n-type ZnO filmprovided on the non-doped ZnO film.
 11. The photodiode according toclaim 6, wherein the buffer layer has an opposite conductivity type tothe light-absorbing layer, and is made of Cds, ZnS, ZnO, ZnMgO, ZnSe, orIn₂S₃.
 12. The photodiode according to claim 6, comprising a mesastructure in configuration of part of the surface layer, the bufferlayer, and the upper electrode layer.
 13. A photodiode comprising: alight-absorbing layer composed of a compound-semiconductor film ofchalcopyrite structure, the compound-semiconductor film of chalcopyritestructure being made of Cu (Inx, Ga1-x) Se2 (0≦x≦1); a surface layerdisposed on the light-absorbing layer, the surface layer having a higherband gap energy than the compound-semiconductor film; an upper electrodelayer disposed on the surface layer; and a lower electrode layerdisposed on a backside of the light-absorbing layer in opposition to theupper electrode layer, wherein the upper electrode layer and the lowerelectrode layer have a reverse bias voltage applied in between to detectelectric charges produced by photoelectric conversion in thecompound-semiconductor film of chalcopyrite structure, and wherein thesurface layer is made of Inx, Ga1-xSe (0≦x≦1), and a particle size ofthe surface layer is smaller than that of the compound-semiconductorfilm of chalcopyrite structure.